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Bluetooth Low Energy for Wireless Sensors and Actuators


There are many wireless protocol options for low-power wireless sensor networks, but none are better suited to the task than Bluetooth low energy.

Wireless solutions are used in a variety of demanding industrial applications. Technologies such as Wireless LAN, Classic Bluetooth, IEEE 802.15.4/ZigBee, and Wireless HART all provide specific characteristics and are therefore suitable for different applications and specific demands. However, none of these technologies offer an optimal solution for a wireless connection for sensors and actuators in manufacturing automation. In these types of applications, the existing technologies are too expensive, too slow, or consume too much energy. The solution lacks a fast, robust, low energy transmission for wireless sensors and actuators. This is where Bluetooth low energy technology comes into play.
Parameter
Bluetooth Wireless LAN ZigBee/IEEE 802.15.4 Bluetooth low energy
Data Throughput Moderate Very Good Not so Good Not so Good
Robustness Very Good Moderate Moderate Very Good
Range 50-1000 m 50-300 m 75 m + mesh 10-300 m
Local system density Very Good Not so Good Moderate Very Good
Roaming Moderate Very Good Not so Good Moderate
Large scale network Not so Good Moderate Very Good Very Good
Low latency Very Good Moderate Not so Good Very Good
Pairing speed Not so Good Moderate Good Very Good
Power consumption Good Not so Good Very Good Very Good
Cost Good Not so Good Good Very Good
Table 1: A comparison of Bluetooth low energy wireless technology and other wireless technologies used in the manufacturing industry.


Bluetooth low energy technology, formerly known as Wibree, was originally developed for the high volume consumer market. Back in June 2007, Nokia and the Bluetooth SIG announced that Wibree would be integrated with Bluetooth. In June 2010, Bluetooth Core Specification v4.0 with the hallmark feature of low energy technology was published. Bluetooth technology now encompasses low energy (Bluetooth v4.0), Classic Bluetooth, and high speed (Bluetooth 3.0 + HS).

Is Bluetooth low energy technology a new technology?

The answer to that question is that the technology is new in some aspects, but not in others. For instance, Bluetooth low energy technology is new in having an efficient discovery and connection setup, very short packets, asymmetrical design for small peripheral devices, and a client- server architecture. But there are also other aspects that are already well established through Classic Bluetooth, such as the Bluetooth radio, HCl logic and physical transport layers, and L2CAP packets.

Thanks to Bluetooth low energy technology, the way we have come to experience wireless is changing. Bluetooth low energy technology brings to the scene the possibility to use wireless in very simple and inexpensive devices, such as the case when integrating sensors. According to the Bluetooth SIG we are looking at potential billions in volumes in the following applications:

  • Phone Accessories: > 10 billion
  • Smart Energy (energy counter and displays): ~ 1 billion
  • Home Automation: > 5 billion
  • Health, Wellness, Sports and Fitness: > 10 billion
  • Assisted Living: > 5 billion
  • Animal Tagging: ~ billion
  • P2P Intelligent Transport Systems: > 1 billion
  • Industrial Automation/M2M: 10 billion
These high volumes and the possibility of integration in mobile phones and laptops allow for low-cost and long-term availability of the radio components.

The technology concept

Possibility for low-power consumption

Bluetooth low energy technology has been designed, from the beginning, to use the lowest possible power consumption. For instance, the Bluetooth low energy unit can be put in sleep mode where it is only used at an event of sending active files to a gateway, PC or mobile phone. Further, the maximum/peak power consumption is set to less than 15 mA and the average power consumption is at about 1 μA. A foundation for the low energy consumption is the very fast connection set-up (few ms) and the short messages. Therefore, the energy consumption is reduced to a tenth of a Classic Bluetooth unit. In other words, a small coin cell — such as a CR2032 — is enough for 5-10 years of operation.

Cost and backwards compatible

In order to be backwards compatible with Classic Bluetooth and to be able to offer an affordable solution for very inexpensive devices, the chipset is available in the following two versions (Figure 1):

  • Dual-mode: Bluetooth low energy technology as well as Classic Bluetooth functionality
  • Stand-alone: Bluetooth low energy technology only in order to optimize cost, power consumption, and size which are particularly useful for small battery powered devices

Figure 1: Bluetooth low energy chipsets are available in two versions.


Robustness, security, and reliability

Bluetooth low energy technology, like Classic Bluetooth, features adaptive frequency hopping in order to secure a robust transmission even in harsh industrial environments. To obtain simpler and cheaper radio chipsets, Bluetooth low energy technology uses only 40 channels, 2 MHz wide while Classic Bluetooth uses 79 channels, 1 MHz wide. (Figure 2).


Figure 2: In the 2.4 GHz band, Bluetooth low energy technology uses 40 channels instead of the 79 channels used in Classic Bluetooth.

Three channels, which are located exactly between the Wireless LAN channels, are used for device discovery and connection setup.

Bluetooth low energy technology has a very secure AES 128 encryption algorithm, and a distributed encryption key procedure.

Coexistence

Bluetooth, Wireless LAN, IEEE 802.15.4/ZigBee, Wireless HART, and many proprietary radios use the unlicensed 2.4 GHz ISM (Industrial Scientific Medical) band. Therefore, in order to get a robust and reliable communication, it is essential for many wireless technologies to make a time consuming and accurate frequency planning. However, Bluetooth technology has already solved these issues thanks to its built-in adaptive frequency hopping feature and high tolerance for interference.


Figure 3: Out of the three advertising channels and the 37 data channels, the three advertising and nine of the data channels are located between the three Wireless LAN channels in the 2.4 GHz band.

These features also make Bluetooth low energy technology coexist smoothly with other wireless technologies in the 2.4 GHz band as Bluetooth technology does not use frequencies that are occupied by other radios in the neighborhood. In addition, there is also a possibility to apply the principle of not using designated channels via the so-called channel blacklisting (Figure 3).

Ease of use and integration

The technology uses a simple star topology, which simplifies the implementation work significantly. This topology fits very well with common used system architecture with a number of smaller devices connected to a master in a production island. In most cases, an Infrastructure/Ethernet network is available and there is no need for mesh networks to extend the geographical coverage.

A unit is always either a master or a slave, but never both. The master communicates with the slaves and it can also communicate simultaneously with multiple slaves. Further, the master controls the timing pattern for the links and the slaves only passes on requests made by the master. A new feature introduced with Bluetooth low energy technology is the advertising function. A unit announces itself by periodically advertising itself. An advertisement can also, for example, include a process value or an event that has occurred (Figure 4).


Figure 4: An advertiser periodically sends and will always act as a slave when it is connecting. A scanner is waiting for an advertisement and is always a master when connecting.

Software structure

In Bluetooth low energy technology, the state, and attributes are key terms. All parameters have a condition available that is provided by the server in a protocol attribute for the client. All attributes have a certain characteristic — signal value, presentation format, etc. — which is described in the client configuration.

In the Generic Attribute Profile (GATT) service groups, attributes, declarations, and descriptions are included. In the Generic Access Profile connection, discoverability, connectable and bonding are described. In this way, a number of basic services and profiles are set up such as for instance timing, battery condition, automation I/O, building automation (temperatures, thermostat, humidity), lighting (On/Off Switch, Dimmer), remote control, fitness (Step Counter, Heart Beat Monitor), medical devices (glucose meters), cars, etc.


Figure 5: Overview of the Bluetooth low energy software structure.

Performance

Connection and latency

Bluetooth low energy technology only uses three channels to build connections and to discover other devices; this not only allows for lower power consumption, but also for a faster connection in only a few ms.

With Bluetooth low energy technology, the latency periods are dependent on how often the master sends messages to the slaves and how often it receives data from the slaves. The latency period for one slave only is 7.5 ms and then increases slowly for each additional slave. For example, with a connection interval of four seconds, a master can address one slave every five ms and thereby achieve updates from hundreds of slaves in only four seconds.

Range

Thanks to a modified modulation, Bluetooth low energy has an approximately 3 dB better link budget compared to Classic Bluetooth. A Bluetooth low energy unit can thereby offer a range of 200-300 meters in line of site without the need of an additional power amplifier. Although industrial sensors and actuators often only need a range of only 20-50 meters, it is important in the aspects of robustness and reliability to have a large reserve in order to bridge temporary obstacles and interference.

Application examples

Based on Bluetooth low energy properties, the technology is very well suited for applications where transferring signal status is important. The examples below show how the I/Os in industrial automation can be used (Figures 6-8).


Figure 6: A portable operator control and monitoring unit (for example, an iPhone) can read and write states of the I/O server.



Figure 7: Both units function as generic I/O clients and are used as generic servers and "indications" to reflect the digital and analog states on the other side.



Figure 8: The I/O unit is a generic I/O server and the fieldbus I/O proxy is a generic I/O client. The I/O device is seen as a field bus I/O from the view of the fieldbus configuration PC. The field bus I/O proxy searches the I/O device attributes database to find out about which digital and analog in and outputs that are available for the I/O-unit.

Another interesting application where the very fast and secure connection is needed is the use of Bluetooth Low Energy as a key in order to allow a mobile operator’s panel to get access to the automation cell or machine.

This feature can also be used in combination with the proximity function. This function is based on the SSI (signal strength indication) value and can give a message if the user is within the production cell or not in order to allow interaction or not.


Figure 9: Automation I/O and proximity are used to control the operator’s access to the manufacturing cell.

Conclusion
  • Cost effective stand-alone solutions
  • Multiple chipset supplier that secure a long-term availability of the components
  • Robustness inherited from Classic Bluetooth — the most robust industrial wireless solution with more than 10 years of experience
  • Long range
  • High local system density
  • Very fast connection
  • Low latency
  • Simple star typology
  • Low-power consumption
  • Free-of-charge technical specifications
  • Test support system through the Bluetooth SIG
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